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The chemistry of battery charging/discharging may be defined closely in a controlled environment but most field applications offer less than ideal conditions. It may be true that a battery may not offgas with any significance if discharged under ideal parameters such as discharge rate (properly matched load), temperature, etc. I would not bet too heavily on that happening in many field applications. For example look at the actual discharge rate of a 14 amp-hr gelcel with a 50watt lamp (typical cave light), and compare it with the manufacturer's recommended parameters. Actually most 14 amp-hr gelcell dive light batteries are two 7 amp-hr batteries in parallel without diode isolation that should be used to prevent one battery from charging the other if there is a performance imbalance between the batteries. However a big question that always needs to be considered is what happens in the event there is a short circuit across the load and the battery discharges at an extreme rate along with all the heat generated in the process? Any battery canister requires some sort of internal overpressure protection (e.g. non-captured o-rings or relief valve). There are many quite amusing stories in the oceanographic community where instrumentation has been destroyed because overpressure relief mechanisms were not incorporated in the systems. This is why you let the engineers familiar with this sort of equipment development the systems, and let the scientists only play with the data. On the opposite end of the explosion question: When ocean engineering students would come to my lab to have their underwater pressure vessels (developed in one of their design classes) tested to failure (i.e. implosion) I always asked them what would be the maximum implodable volumn allowed for an underwater housing if it was located in a critical application (e.g. next to a diver, near a viewport on a submersible, or near critical equipment/instrumentation). The point being that these sorts of considerations are just as important as the actual mechanics of the system. If the system is located in a critical location, such as next to a diver, then additional precaution must be exercised to ensure safety. IMHO, Doug -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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